Oxygen sensors

ABSTRACT

An electrochemical oxygen sensor includes a micro-porous plastic membrane supported on a sealing disk and located between a gas inflow port and the sensor&#39;s electrolyte. The membrane and disk minimize thermal shock effects due to using the sensor at a first location, at a first temperature, and then moving it to a second location at a different temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This continuation application claims the benefit of the filing date ofparent U.S. patent application Ser. No. 11/877,331 filed Oct. 23, 2007,entitled “Oxygen Sensors”, incorporated herein by reference, whichclaimed the benefit of the filing date of U.S. Provisional ApplicationSer. No. 60/863,859 filed Nov. 1, 2006 and entitled “Oxygen Sensors”.

FIELD

The invention pertains to electrochemical gas sensors. Moreparticularly, the invention pertains to electrochemical oxygen sensorswhich are resistant to thermal shock.

BACKGROUND

Electrochemical sensors are known and can be used to detect varioustypes of gases including oxygen as well as other types of gases.

Representative sensors have been disclosed in U.S. Pat. No. 5,668,302 toFinbow et al. entitled Electrochemical Gas Sensor Assembly, issued Sep.16, 1997, and U.S. Pat. No. 5,746,899 to Finbow et al. entitledElectrochemical Gas Sensor, issued May 5, 1998. The '302 and '899patents have been assigned to the assignee hereof and are incorporatedby reference. Useful as they have become, such sensors are not withoutsome limitations.

A fairly common problem experienced by users of portable oxygen gasdetection equipment is that the instrument can be susceptible to thermalshock and generate false alarms when the user moves between locations atdifferent temperature. Typical conditions that might generate the falsealarm condition would be when the user exits a heated office orcalibration station into a cold working environment.

This situation is most noticeable in winter when the temperaturedifference often exceeds 30° C. Whilst the effect is often associatedwith a negative temperature change i.e. movement from a warm to coolerenvironment, the same effect can also manifest itself in the oppositesense when there is a positive temperature change.

Thermal shock in an oxygen sensor or cell is usually characterized by arapid change in output response other than that caused by normaldiffusion, when a change in temperature is experienced by the cell.Thermal shock does not always happen immediately and thermal shocks havebeen noticed after time periods of over one hour after the initialtemperature change occurred. This has implications in a finished productof false alarms where a false oxygen level is registered by the cell.

The cause of the problem is related to the design and construction ofthe oxygen sensor which relies on controlled diffusion of oxygen intothe sensor from the external environment via a capillary hole. Onceoxygen has entered the cell it reacts and generates a current that isproportional to the oxygen concentration in the external environment.

Large temperature excursions can cause an additional contribution to thesignal when the internal cell pressure (caused by the temperaturechange) equilibrates with the environment. The causes of these pressuredifferences include air, or, gas pockets within the body of the cellwhich expand or contract with temperature.

For the typical condition described above, the gas inside the cellcontracts when the instrument is transferred to the cold environment.The pressure difference caused through contraction draws air into thecell through the capillary leading to an enhanced cell output and falsealarm.

Thus, there continues to be a need for improved oxygen sensors whichminimize false alarms. Preferably such improved functionality could beachieved without substantially increasing the manufacturing complexityand cost of such units.

Also, it would be preferable if such improved detectors could beimplemented as portable or human wearable to facilitate use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a sealing disk according to one embodiment ofthe present invention;

FIG. 2 illustrates a side elevational view of a sealing disk as in FIG.1;

FIG. 2A is an enlarged partial view of an edge of the disk of FIG. 2;

FIG. 2B is an enlarged partial view of a central region of the disk ofFIG. 2; and

FIG. 3 is an exploded view that illustrates the components of an oxygencell according to one embodiment of the present invention.

DETAILED DESCRIPTION

While embodiments of this invention can take many different forms,specific embodiments thereof are shown in the drawings and will bedescribed herein in detail with the understanding that the presentdisclosure is to be considered as an exemplification of the principlesof the invention, as well as the best mode of practicing same, and isnot intended to limit the invention to the specific embodimentillustrated.

Embodiments of the invention solve the thermal shock problem by creatinga partition in the cell that prevents bulk flow or exchange of airbetween the cell and the environment. In a preferred embodiment of theinvention, the partition comprises a micro-porous plastic membranesupported on a compressible foam adhesive gasket which is itselfsupported on a rigid plastic perforated disk.

The disk serves a number of functions. The primary role is to supportthe foam gasket and membrane. Additionally, an air tight seal can beformed between the plastic disk and the cell body or housing. This isachieved, in a disclosed embodiment, by ultrasonically welding theplastic disk to the main outer plastic body of the cell. The weld alsoserves to form an air tight seal around the metallic current collectorstrip that passes between both compartments within the cell.

It is preferable that the membrane be adequately supported and not beable to deform or flex under pressure, since this behavior can alsogenerate pressure transient effects of sufficient magnitude to causefalse alarms. The disk can be annular with a hole in the middle to allowelectrolyte ions to pass through the partition between the upper andlower compartments of the cell thereby facilitating normal operation ofthe cell. Alternatively the single central hole in the disk could bereplaced with a plurality of smaller holes, each also capable ofallowing electrolyte transport across the disk to the electrode from themain body of the cell.

The function of the micro-porous membrane is to prevent the bulktransport of gas (usually in the form of bubbles) through the cell. Itrelies on the fact that liquid is held in the pores of the membrane bysurface tension and capillary action, and that pressure must be appliedto overcome these forces before bubbles/air are able to cross themembrane. The minimum pressure (bubble point pressure) required to forcethe liquid out of the membrane pores is related to the membranecapillary geometry according to the following equation; P=(4k cos θσ)/dwhere P is the bubble point pressure, d is the pore diameter, k is ageometry correction factor, θ is the liquid solid contact angle and σ isthe surface tension. Therefore if a membrane material is chosen withpores of sufficiently small diameter, the pressure required for gas tocross the membrane will exceed that created by environmental temperaturechanges, thereby eliminating the temperature transient effect.

An estimate of the pressure difference that the membrane needs towithstand to eliminate the problem can be calculated from the gas law(PV=nRT). Where P, V, T, and n are pressure, volume, temperature, andamount of gas with R as the gas constant. For a 40° C. temperaturereduction the internal cell pressure decreases by 13.3% which isequivalent to 0.133 bar. Therefore a suitable membrane material can beexpected to have a minimum bubble point pressure of at least 0.13 bar.In practice materials are chosen with values that exceed this.

The compressible foam gasket serves two purposes. The adhesive surfacesensure an air tight seal between the supporting shelf and the membranematerial; in addition the compressible nature of the material ensuresthat any “dead volume” in the upper partition is minimized. The amountof free “dead volume” in the upper partition is associated with the sizeof the initial thermal transient that all lead based oxygen sensors showon rapid changes of temperature.

Embodiments of the invention include a micro-porous plastic membranematerial. The membrane material is not swelled or deformed byelectrolyte, nor chemically degraded by electrolyte, or reactionproducts of oxygen reduction.

The Micro-pores in the membrane also allow transport of ions through thefilm unlike some solid membranes which only allow water migration.Therefore the membrane material does not promote osmosis which may undercertain circumstances prove to be an issue.

Use of a plastic support disk, or plate, as in FIG. 1, which can beultrasonically welded to a cell body to provide air tight seal with thebody and current collector, improves cell reliability and life-time. Thedisk also stops the membrane from flexing under pressure. Use of acompressible gasket minimizes “dead volume” in a cap or cover for thecell thereby reducing initial thermal transient effects.

FIGS. 1, 2-2B illustrate various aspects of a disk or partition 12 inaccordance with the invention. As illustrated therein, disk 12 is arigid, generally annular shaped member with first and second spacedapart planar surfaces 12-1, 12-2 bounded by a peripheral support 12-3.The disk 12 has a central opening 12-4. The disk 12 carries acompressible foam adhesive gasket 14. The gasket in turn carries aselected micro-porous plastic membrane 16.

As noted above, the disk 12 can be ultrasonically welded to an externalhousing of a respective oxygen sensor. As will be understood by those ofskill in the art, disk 12 could alternately be perforated by one or moreopenings therethrough. The openings need not be centrally located, butcould be distributed across the disk. Alternately, disk 12 could beformed of a material permeable to a selected electrolyte.

FIG. 3 is an exploded view of a representative oxygen sensor 30 whichembodies the present invention. The cell 30 includes a hollow,cylindrical body 32 which defines an interior region 34 for anelectrolyte for the cell.

A top end 36 of the body 32 defines an annular region indicatedgenerally at 38 which can receive and support a multiple elementseparator filter 46.

The sealing disk 12 as previously discussed, is supported by the annularsurface 38 and can be ultrasonically welded to the body 32. Thecompressible foam adhesive gasket 14, also annular in shape, having acentral opening 14 a, overlays and is supported by the disk 12. Filter18 fills the opening 14 a of the gasket 14.

Membrane 16 overlays the gasket 14. The cell 30 is closed with a cap 40which could be affixed to the body 32 by welding or adhesive. The cap 40can carry a working electrode 42. The body 32 can carry an internalcurrent collector element 44.

In assembling one embodiment of the present invention, as in FIG. 3, theplurality of separator filters 46 is located centrally on the top of thecell body molding 32. The plastic sealing disk, 12, as in FIG. 1 forexample, is then placed centrally within the recess 38 of the bodymolding 32 on top of the separator filters 46 ensuring that the internalcurrent collector 44 is located between the plastic sealing disc 12 andbody molding 32. The sealing disc 12 is then ultrasonically welded tothe body molding 32.

Partition 10 is a multi-component separator structure. An adhesive foamannulus 14, such as a closed cell EPDM, is located centrally on theplastic sealing disk 12. A plurality of small separator filters 18 isthen located in the opening 14 a formed by the annular foam gasket 14and plastic sealing disc 12. The micro-porous membrane 16 is locatedcentrally on top of the adhesive foam disc or gasket 14.

The internal current collector 44 is then folded over the nylon ionicmembrane 16. The pre-assembled cap molding 40 and working electrode 42is then placed on to the cell body assembly 32 and ultrasonically weldedinto place to complete cell 30.

Embodiments of the invention remove thermal shock within an oxygen cellby sealing the lead chamber of the cell from the working electrodepreventing gas transfer between the two. Thermal transients (The initialpeak in cell output when exposed to a sudden reduction in temperature)can be reduced by removing free volume and therefore trapped gas in thetop half of the cell within the area between ionic membrane and workingelectrode.

By using a plastic disc such as disk 12 as a support for a nylonmembrane, such as membrane 16 and welding the disc to the body 32 of theoxygen cell such as cell 30 it is possible to create a gas tight sealbetween upper and lower parts of the cell and remove thermal shockeffects that are caused by transfer of gas between the two parts.Further, by using an EPDM Closed foam gasket, such as gasket 14, betweenthe plastic disc 12 and membrane 16 it is possible to lower the amountof free volume in the top part of the cell 30 and therefore lower theinitial thermal transient.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the invention. It is to be understood that no limitation withrespect to the specific apparatus illustrated herein is intended orshould be inferred. It is, of course, intended to cover by the appendedclaims all such modifications as fall within the scope of the claims.

The invention claimed is:
 1. An electrochemical gas sensor comprising: ahousing with an open end; a partition carried on the open end with thepartition including at least a rigid support member that carries aselected membrane, the support member is sandwiched between the membraneand the open end of the housing, and the support member is sealed to theopen end of the housing with an air tight seal; a cap which carries anelectrode, the cap is sealed to the housing adjacent to the open end;and a current collector which extends from within the housing, throughthe seal between the end of the housing and the support member and intoa region between the electrode carried by the cap and the membranewherein the membrane further comprises liquid held in the pores of themembrane, the pores having a diameter that requires a predeterminedminimum pressure to force the liquid out of the pores and before gas isable to cross the membrane, the minimum pressure exceeding that createdby environmental temperature changes thereby preventing the bulktransport of gas across the membrane and eliminating temperaturetransient effect.
 2. An electrochemical gas sensor as in claim 1 whichincludes sensing and counter electrodes.
 3. An electrochemical gassensor as in claim 1 where the membrane is positioned in the sensorbetween a counter electrode and a gas inflow port.
 4. An electrochemicalgas sensor as in claim 3 which includes a compressible gasket whichcarries the membrane.
 5. An electrochemical gas sensor as in claim 1which includes a compressible foam adhesive gasket which carries themembrane, the gasket forms another air tight seal.
 6. An electrochemicalgas sensor as in claim 1 where the support member defines at least oneopening therethrough.
 7. An electrochemical gas sensor as in claim 6which includes a compressible gasket which carries the membrane, thegasket overlays the support member, and, the gasket reduces the amountof free volume in a region between the support member and the cap.
 8. Anelectrochemical gas sensor as in claim 7 where the cap has a gas inflowport, the cap being adjacent to the membrane, the support member beingadjacent to an internal region of the housing and to an electrode.
 9. Anelectrochemical gas sensor as in claim 7 where the gasket includesadhesive surfaces which provide another air tight seal between thesupport member and the membrane.
 10. An electrochemical gas sensor as inclaim 9 where electrolyte is adjacent to the support member and locatedwithin the housing.
 11. An electrochemical gas sensor as in claim 10where the support member is one of perforated, or, permeable to theelectrolyte.
 12. A gas sensor which comprises a housing having first andsecond chambers; the housing carrying at least sensing and counterelectrodes; a body of electrolyte in contact with at least one of theelectrodes; the housing defining a gas inflow port; a planar membranelocated within an interior of the housing, closing the interior of thehousing between the first and second chambers, to regulate an inflow ofambient gas, the membrane allowing electrolyte to pass therethrough andpreventing an exchange of air between the first and second chambers, themembrane holding liquid in the pores thereof by surface tension andcapillary action; an air tight seal between one of the chambers and arigid support member that underlies the membrane, and a second air tightseal between at least portions of the membrane and the rigid supportmember wherein the pores have a diameter that requires a predeterminedminimum pressure to force the liquid out of the pores and before gas isable to cross the membrane, the minimum pressure exceeding that createdby environmental temperature changes thereby preventing the bulktransport of gas across the membrane and eliminating temperaturetransient effect.
 13. A gas sensor as in claim 12 which includes agasket, where the gasket carries the membrane and implements the secondair tight seal, at least in part.
 14. A gas sensor as in claim 13 whichincludes a current collector which extends between both chambers of thehousing, through a portion of the air tight seal, wherein the seal isformed by welding the support member to the housing.
 15. An oxygensensor as in claim 14 where a portion of the current collector extendsinto a region adjacent to the membrane.
 16. A gas sensor as in claim 13where the gasket comprises an adhesive, compressible gasket whichimplements the second air tight seal.
 17. An oxygen sensor as in claim16 where the support member comprises one of an annular disk, a diskexhibiting a plurality of spaced apart openings therethrough, or anelectrolyte permeable disk.
 18. An electrochemical gas sensor comprisinga housing with a gas inflow port, the housing carries at least oneelectrode and which includes an inwardly extending, at least partlyannular support surface for a support element, a gasket and a membranewhere the gasket is located between the at least one electrode and thesupport element, and the gasket provides at least in part an air tightseal between the membrane and the support element; and at least onefilter element between the support element and the at least partlyannular support surface wherein the membrane further comprises liquidheld in the pores of the membrane, the pores having a diameter thatrequires a predetermined minimum pressure to force the liquid out of thepores and before gas is able to cross the membrane, the minimum pressureexceeding that created by environmental temperature changes therebypreventing the bulk transport of gas across the membrane and eliminatingtemperature transient effect.
 19. An electrochemical gas sensor as inclaim 18 which includes a second air tight seal between the supportelement and the at least partly annular support surface.
 20. Anelectrochemical gas sensor as in claim 19 which includes a currentcollector which extends in the housing, through a portion of the airtight seal, wherein the seal is formed by welding the support element tothe housing.